“Molecular sieve” means both of aluminosilicate-type zeolite and non-aluminosilicate-type zeolite-like materials such as pure silicates, silicoaluminophosphates or borosilicates.
Zeolite has versatile utility due to its structural features (channels and cavities) that uniform micropores (0.3 nm<diameter<2 nm) of molecular dimension are regularly arranged. One of the most important industrial applications is the role as nonhomogeneous acidic catalyst under acifified condition. The majority of the world's gasoline is currently produced by the fluidized catalytic cracking of petroleum using zeolite catalyst (Cundy, C. S. et al., Chem. Rev., 2003, 103, 663).
In addition to natural zeolite, synthetic zeolite and zeolite-like materials have various structures and properties, and therefore, they can be more widely utilized as ion exchange materials, adsorbents, and catalyst. Since the micropore diameter and structure in a molecular sieve constitute important factors to control adsorption ability and catalytic ability, syntheses of molecular sieves having a new structure are actively attempted in order to improve the adsorption ability and catalytic ability of molecular sieves. In rescent, molecular sieves which micropore sizes are extended to 2 nm˜50 nm have been developed, but they are non-crystalline and have very different properties as compared with those of crystalline molecular sieves. As a result, their utilization is low.
The molecule diffusion rate in zeolite is very low due to its microporous structure, which restricts a reaction rate in many applications. Therefore, there have been attempts to improve the molecule diffusion into micropores by increasing the outer surface area of zeolite particles itself, and thereby facilitating the approach of molecules to the micropores.
At first, there have been attempts to synthesize zeolite in a very small particle size in order to synthesize zeolite particles possessing a wide outer surface area (e.g., Valtchev, V. P. et al., Chem. Mater., 2005, 17, 2494). The small particle size was suggested to offer the advantage of facile diffusion of reactants. However, filtration of the small zeolite particles was not easily achievable due to the colloidal properties. Ul-tracentrifugation was commonly used for the sample collection, which significantly increased the cost of bulk production.
Next, several attempts to synthesize microporous zeolites possessing secondary mesopores (2 nm<diameter<50 nm) were reported to overcome the problem. Anderson et al. reported a preparation method of zeolite materials possessing large secondary pores, through a crystallization process utilizing the infiltration of zeolite seed crystals into mesoporous diatomaceous earth (Anderson, M. W. et al., Angew. Chem. Int. Ed., 2000, 39, 2707). Pinnavaia et al. also carried out a self assembly of pre-formed zeolite seed crystals in the presence of surfactant to prepare mesoporous materials (U.S. Pat. No. 6,770,258 B2). Kaliaguine et al. coated the mesopore walls of pre-synthesized mesoporous silica with zeolite seed crystals (U.S. Pat. No. 6,669,924 B1). The resultant materials synthesized with these strategies were claimed to exhibit enhanced hydrothermal stability, compared with amorphous aluminosilicate materials having similarly mesoporous structure. It was also claimed that molecular diffusion took place rapidly, compared with the absence of mesopores. However, no direct evidence by X-ray diffraction (XRD) or electron microscopy was supported as to the structure comprising crystalline zeolite. Furthermore, the use of pre-formed zeolite seed crystals was a problem of complicating the overall synthesis process and thereby increasing the cost of production.
In recent, mesoporous zeolites were also prepared by crystallization in the presence of various solid templates such as carbon nanoparticles, nanofibers and polymer beads. Zeolites crystallization occurred across the template particles, and combustion of the template particles led to the formation of mesopores within the resultant zeolite crystals. Stein et al. reported a technology wherein polystyrene beads having a uniform size around 100 microns could direct the formation of mesoporous silicalite-1 (U.S. Pat. No. 6,680,013 B1). Jacobson synthesized mesoporous zeolites with a wide pore-size distribution of 10-100 nm by using carbon black particles as a template. (U.S. Pat. No. 6,620,402 B2). More recently, Kaneko and coworkers synthesized mesoporous ZSM-5 monolith with a narrow pore size distribution via similar carbon templating method by using nano-sized carbon aerogel as a template (Kaneko, K. et al., J. Am. Chem. Soc., 2003, 125, 6044). The resulting materials prepared from the templating methods exhibited XRD patterns corresponding to the particular zeolite structures. The templated zeolites were reported to exhibit an enhanced catalytic activity due to the facile molecular diffusion via the mesopores (Christensen, C. H. et al., J. Am. Chem. Soc., 2003, 125, 13370). However, the solid-templating methods required use of a suitable template material, and more importantly, a precise control of process was required so that the crystallization could occur selectively within the templating zone. This was a major factor for complicating the preparation process and increasing the production cost.
Zeolite materials having both of micropores and mesoposes in a particle have multiple advantages. The intrinsic micropores in the zeolite framework provide with molecule selectivity and active sites and the additional mesopores facilitate the molecule diffusion within micropores to improve the diffusion and adsorption of molecules as well as to modify the diffusion and adsorption of even larger molecules.